This application claims priority to an application entitled xe2x80x9cDevice and Method for Generating Short PN Code in Mobile Communication Systemxe2x80x9d filed in the Korean Industrial Property Office on Nov. 2, 1998 and assigned Ser. No. 98-47240, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to a short PN code generation device and method for a CDMA mobile communication system, and, in particular, to a device and method for generating a short PN code for a high-speed mobile communication system.
2. Description of the Related Art
In general, CDMA (Code Division Multiple Access) mobile communication systems use orthogonal codes, long PN (Pseudo Noise) codes and short PN codes. The short PN codes are used to identify the base stations. For example, the forward link of the IS-95/IS-95A CDMA communication system identifies the base stations by using offsets to short PN codes.
The IS-95/IS-95A forward link uses a short PN code of length 215 to identify the base stations. The respective base stations and mobile stations generate the short PN code of length 215 in such a manner that while one base station generates the t-th chip of the short PN code, another base station generates a (t+(64*k))-th chip of the short PN code. A xe2x80x9cchipxe2x80x9d is an individual binary digit in the short PN code. Herein, xe2x80x9c(64*k)xe2x80x9d will be referred to as an offset. Therefore, the base stations use unique short PN codes each having different offset values to identify themselves. The mobile stations then identify the base stations by searching for the offsets of the short PN codes from the base stations. That is, the mobile stations identify the base stations depending on the offset values of the received short PN codes.
FIG. 1 shows a base station device for spreading a transmission signal using a short PN code of length 215 in a conventional CDMA mobile communication system. In. FIG. 1, the finally output code unit is a chip. The existing CDMA mobile communication system generates 1.2288M chips per second for the short PN code, and the clock rate of the generated short PN code is equal to a chip rate.
Referring to FIG. 1, multipliers 111 and 113 commonly receive an orthogonally spread signal which was multiplied by an orthogonal code (such as a Walsh code). The multiplier 111 multiplies an I-component short PN code by the orthogonally spread signal, and the multiplier 113 multiplies a Q-component short PN code by the orthogonally spread signal. Here, the I-component short PN code and the Q-component short PN code are created so that the base station has a unique offset. A baseband filter 115 filters a baseband signal from the I-channel spread signal output from the multiplier 111, and a baseband filter 117 filters a baseband signal from the Q-channel spread signal output from the multiplier 113. A mixer 119 mixes an output of the baseband filter 115 with a carrier cos(2xcfx80fct), and a mixer 121 mixes an output of the baseband filter 117 with a carrier sin(2xcfx80fct). An adder 123 adds an output of the mixer 119 and an output of the mixer 121.
However, in an IMT-2000 system, which is a third generation mobile communication system introducing a multi-carrier concept, it is expected that the chip rate will increase. With regard to the chip rate, the IS-95 communication system is termed a 1xc3x97 system, while the IMT-2000 communication system can be extended up to a 3xc3x97 system, a 6xc3x97 system, a 9xc3x97 system and a 12xc3x97 system, including the legacy 1xc3x97 system. In terms of chips per second, the 1xc3x97 system has a chip rate of 1.2288 Mcps (chips per second), the 3xc3x97 system has a chip rate of 3xc3x971.2288 Mcps=3.6864 Mcps, the 6xc3x97 system has a chip rate of 6xc3x971.2288 Mcps=7.3728 Mcps, the 9xc3x97 system has a chip rate of 9xc3x971.2288 Mcps=11.0592 Mcps, and the 12xc3x97 system has a chip rate of 12xc3x971.2288 Mcps=14.7456 Mcps. Accordingly, the 1xc3x97 system operates at the chip rate as that of the existing CDMA mobile communication system, whereas other Nxc3x97 systems operate at a chip rate increased by N times. At this point, in the Nxc3x97 systems, the time period of each chip is decreased by N times. Therefore, if the Nxc3x97 system uses the same offset as in the 1xc3x97 system, the time period of the offset will be decreased by N times. Accordingly, it is difficult for the mobile station to identify the base stations because it is hard to distinguish whether the short PN code signal is a multipath signal from the existing base station or a short PN code signal from another base station. Thus, it is required to increase the offset by N times. Therefore, if the Nxc3x97 system uses the short PN code having the same period as the short PN code used in the existing 1xc3x97 system, the number of cases (or the number of offsets) for identifying the base stations is decreased by N times. Accordingly, there is required a short PN code of time period Nxc3x97215 in order to have the same offset number as the existing CDMA mobile communication system.
The above stated short PN code is a PN sequence having a period of 2k. The 3xc3x97 system requires a short PN code of period 3xc3x97215. However, there exists no PN sequence having this period. Up to date, a sequence having the 3 random sequence properties and satisfying the PN sequence having this period has not been found (see xe2x80x9cShift Register Sequencesxe2x80x9d by Solomon W. Golomb).
Therefore, there has been research on a method of generating a PN sequence of period 217, and then using only part of the 217 PN sequence in order to obtain the short PN code of period 3xc3x97215. Since the short PN codes of period 3xc3x97215 generated by this method do not have the 3 properties of the random sequences, every effort has been made to reduce the number of hardware gates in generating the I-component short PN code and the Q-component short PN code.
As described above, the short PN code used in the IS-95 system cannot be used in the IMT-2000 mobile communication system due to the decrease in size of the offsets. In addition, when the IMT-2000 communication system uses the same short PN code as the IS-95 system, an increased number of hardware gates are required to generate the short PN code.
It is, therefore, an object of the present invention to provide a device and method for generating I-component and Q-component short PN codes using a single short PN code generator in a CDMA mobile communication system.
It is another object of the present invention to provide a device and method for generating I-component and Q-component short PN codes by alternately switching a PN code generated from a single short PN code generator in a CDMA mobile communication system.
It is further another object of the present invention to provide a device and method for generating a short PN code that is longer in length than the short PN code used in an IS-95 system to generate I-component and Q-component short PN codes using the generated short PN code in a CDMA mobile communication system.
In accordance with one embodiment of the present invention, there is provided a short PN code generating device in a mobile communication network having a Nxc3x97 CDMA communication system performing spreading at an Nxc3x97 chip rate using a short PN code of chip length Nxc3x972k and a 1xc3x97 CDMA communication system performing spreading at a 1xc3x97 chip rate using a short PN code of chip length 2k, wherein the device generates a short PN code such that the Nxc3x97 CDMA communication system can distinguish at least the same number of base stations as the 1xc3x97 CDMA communication system can distinguish. The device comprises a sequence generator for generating an m-sequence having a chip length longer than the chip length Nxc3x972k to output an I-component short PN code; and a Q-component short PN code generator for logically operating the parallel outputs of the sequence generator with associated masks and XORing the logically operated values to generate a Q-component short PN code, wherein the masks have specific values so that the Q-component short PN code maintains a predetermined chip period distance from the I-component short PN code.
In accordance with another embodiment of the present invention, there is provided a short PN code generating device in a mobile communication network having a Nxc3x97 CDMA communication system performing spreading at an Nxc3x97 chip rate using a short PN code of chip length Nxc3x972k and a 1xc3x97 CDMA communication system performing spreading at a 1xc3x97 chip rate using a short PN code of chip length 2k, wherein the device generates a short PN code such that the Nxc3x97 CDMA communication system can distinguish at least the same number of base stations as the 1xc3x97 CDMA communication system can distinguish. The device comprises a sequence generator for generating an m-sequence having a chip length longer than the chip length Nxc3x972k in order to output an I-component short PN code; and a short PN code generator for switching the generated m-sequence at predetermined chip periods in order to alternately output the m-sequence as an I-component short PN code and a Q-component short PN code.